1. Field of the Invention
The present invention relates to a method of monitoring progress of polishing of a substrate, such as a semiconductor wafer, and more particularly to a method of monitoring progress of substrate polishing based on a change in spectrum obtained from reflected light from the substrate and determining a polishing end point.
The present invention also relates to a polishing monitoring apparatus for performing such a polishing monitoring method.
The present invention further relates to a polishing method for a substrate using such a polishing monitoring method.
2. Description of the Related Art
In fabrication processes of a semiconductor device, several kinds of materials are repeatedly deposited in the form of film on a silicon wafer to form a multilayer structure. It is important for forming such a multilayer structure to planarize a surface of a top layer. Chemical mechanical polishing (CMP) is widely used as one of techniques for achieving such planarization.
The chemical mechanical polishing (CMP) is performed by a polishing apparatus. The polishing apparatus of this type typically includes a polishing table supporting a polishing pad thereon, a top ring for holding a substrate (a wafer with a film formed thereon), and a polishing liquid supply mechanism for supplying a polishing liquid onto the polishing pad. Polishing of a substrate is performed as follows. The top ring presses a surface of the substrate against the polishing pad, while the polishing liquid supply mechanism supplies the polishing liquid onto the polishing pad. In this state, the top ring and the polishing table are rotated independently to provide relative movement between the substrate and the polishing pad, thereby polishing the film that forms the surface of the substrate.
Generally, the polishing apparatus has a polishing end point detection device. An optical polishing end point detection device is one example of such a polishing end point detection device. This device is configured to direct light to the surface of the substrate and to determine a polishing end point based on spectrum of the light reflected from the substrate. For example, a Japanese laid-open patent publication No. 2004-154928 discloses a method in which intensity of the reflected light is processed in order to remove noise components to create characteristic value and the polishing end point is determined based on a distinctive point (i.e., a local maximum point or local minimum point) of temporal variation in the characteristic value.
The spectrum is an arrangement of the light intensity in the order of wavelength. The characteristic value created from the spectrum varies periodically with polishing time, as shown in FIG. 1, and the local maximum point and the local minimum point appear alternately. This phenomenon is due to interference between light waves. Specifically, the light, directed to the substrate, is reflected off an interface between a medium and the film and an interface between the film and a layer beneath the film. The light waves from these interfaces interfere with each other. The manner of interference between the light waves varies depending on the thickness of the film (i.e., a length of an optical path). Therefore, the intensity of the reflected light from the substrate varies periodically in accordance with the thickness of the film. The intensity of the light can also be expressed as a relative value, such as a reflectance or a relative reflectance.
The above-described optical polishing end point detection device counts the number of distinctive points (i.e., the local maximum points or local minimum points) of the temporal variation in the characteristic value during polishing and monitors the polishing progress based on the number of distinctive points. The polishing process is terminated when a predetermined period of time has elapsed from a point of time when the number of distinctive points has reached a predetermined value.
There is also a method of determining the polishing end point by comparing spectrum obtained during polishing with reference spectrum that is prepared in advance, as disclosed in a Japanese laid-open patent publication No. 2009-505847. In this method, the spectrum at each point of time during polishing is compared with the reference spectrum. A point of time when a difference between both spectra satisfies a condition of a target difference is determined to be the polishing end point. The reference spectrum is prepared in advance by polishing a sample substrate of the same type as the substrate to be polished.
A plurality of spectra including the reference spectrum, which are obtained during polishing of the sample substrate, are associated with index values that are correlated with polishing time and rotational speed of the polishing table. The spectra thus obtained are stored as library. Therefore, by comparing spectrum obtained during polishing of another substrate with the spectrum in the library, a polished state of the substrate at each point of time during polishing can be expressed by the index value. This index value can be defined as an index that indicates a film thickness of the substrate relatively or indirectly.
However, in an actual substrate, multiple interconnect layers with different interconnect patterns and multiple dielectric films of different types are piled up to form multilayer interconnect structure. The optical sensor detects the light reflected from a lower dielectric film through a non-interconnect portion of an upper dielectric film. Consequently, if there are variations in thickness of the lower dielectric film and optical constant between substrates, the spectrum is affected by these variations. As a result, the above-described method cannot accurately measure the thickness of the upper film (i.e., the film to be polished) and it is therefore difficult to accurately monitor the progress of polishing. Furthermore, the variations in thickness of the lower dielectric film and optical constant would result in differences in the detected polishing end point between the substrates.